The present invention relates to a device for extruding honeycomb structures which are used as catalyst supports, heat exchangers or filters.
Conventionally, the mechanical strength of the whole of the ceramic honeycomb structure or of the outer peripheral portion thereof has been improved by forming an outer wall B having a thicker wall than that of a grid-shaped wall A, in the outer periphery of the honeycomb structure as shown in FIG. 1.
For extruding such a honeycomb structure as described above, devices as shown in FIGS. 2 and 3 are conventionally used.
These devices are provided with a die mask 1 having an inner surface 6 of a smaller diameter than an extrusion die 3, in the outer periphery of the outlet side of the extrusion die 3 disposed within a cylinder 2 respectively.
The extrudable material which was supplied from the cylinder 2 passes feed passageways 4 and grid-shaped extrusion slots 5 which are formed within the die 3, under pressure to be extruded therefrom.
Then, the extruded body is formed by the inner wall 6 of the die mask 1 into a predetermined outer form and the thick outer wall is also formed.
However, according to the device of FIG. 2, the material is pressed by the tapered inner surface 6 of the die mask 1 toward the center thereof after being extruded out of the die 3. Therefore, distortion is easy to occur in the connecting portion C between the grid-shaped wall A and the outer wall B of the obtained honeycomb structure as shown in FIG. 4. As a result, the mechanical strength of the connecting portion C is decreased.
And in the device of FIG. 3, the die mask 1 which is contacted with the die 3 is provided with an opening portion 30 in the whole circumference of the inner wall 6 thereof.
In the outer periphery of the obtained product, the thick outer wall is formed by the material which is supplied from the extrusion slots of the outer peripheral portion of the die 3 through the opening portion 30.
However, according to the device of FIG. 3, since the connecting portion C shown in FIG. 4 is crushed by the material supplied to the outer periphery of the extruded body, distortion is easy to occur therein.
Furthermore, since the thickness of the outer wall of the honeycomb structure is largely different from that of the grid-shaped wall thereof, the heat transfer from the grid-shaped wall portion into the outer wall becomes discontinuous in the connecting portion therebetween, or the inclination of the heat transfer abruptly changes therein when the conventional honeycomb structure is used in the exhaust gas purifying device of an automobile. As a result, the honeycomb structure is easy to be broken in the connecting portion.
These drawbacks of the honeycomb structure are inevitable when it is extruded by the devices shown in FIG. 2 and FIG. 3. And these drawbacks are caused by the defects of the conventional extrusion devices.
The present inventors in part and their coinventor Okumura have proposed one honeycomb structure as shown in FIG. 5 in U.S. Pat. No. 4,233,351 wherein the defects of the conventional honeycomb structure can be overcome.
In the honeycomb structure shown in FIG. 5, the wall thickness of each open passage positioned in the outer peripheral portion thereof is larger than that of each open passages positioned in the central portion thereof.
This honeycomb structure has a sufficiently larger strength against thermal shock compared with the conventional honeycomb structure of which only the outer wall has an extremely large thickness.
Accordingly, one object of the present invention is to provide an extrusion device for producing honeycomb structures having an excellent mechanical strength and thermal shock resistance.
Another object of the present invention is to provide an extrusion die for producing honeycomb structures wherein the wall thickness of the open passages positioned in the outer peripheral portion thereof is larger than that of those positioned in the central portion thereof and no distortion is observed in the wall of any open passages.